Discovery: Evidence is accumulating that Mars’s climate undergoes dramatic periodic changes and may now be in a warming trend. The changes appear to be driven by large oscillations in Mars’s orbit and axial tilt.

Above is a radar cross-section of Mars’s north polar ice cap. To the right is the pattern of sedimentary layering in Becquerel crater, in the Arabia Terra region. Both ice and sedimentary deposits display significantly periodic layering patterns. Ice layering in the north polar region shows rhythmic cycling between bundles of dust-containing layers and interspersed clean ice, reflecting changes in environmental conditions during deposition. In Becquerel crater, sedimentary layering shows semi-regular cyclic bedding, again suggesting periodic environmental changes. In geologic terms the ice caps were deposited quickly and recently, the sedimentary layers over long, much older periods; the patterns suggest that both short and long-period cycles may be involved, perhaps on the order of 104-107 years.

The driving force for the climate changes appears to be the large variations in Mars’s planetary motions. As with Earth, Mars’s orbit is eccentric, its rotational axis precesses, and especially, its obliquity (axial tilt) oscillates; the martian variations are all larger than the terrestrial, partly because Earth is anchored by the Moon. Mars’s obliquity ranges between at least 15º and 35º, possibly as widely as 0º-60º, compared to the Earth’s roughly 22º-24º. At low obliquity, less sunlight falls on polar regions, which accumulate snow. At high obliquity the poles receive more sunlight and the equator less, so snow migrates to equatorial regions. At present the obliquity of Mars is calculated to be roughly 25º and decreasing, indicating that in (geologically) recent times the axial tilt was substantially larger and ice would be expected near the equator.

From Head et al. (2008). Copyright 2008 National Academy of Sciences, U.S.A. Used by permission.

Graphic: NASA/JPL/Los Alamos National Laboratory

The suggestion that Mars is emerging from a period of ice in temperate latitudes agrees with MRO’s detection of remnant glaciers there. Widespread near-surface ice and crater gullies, some of which may have formed as snowpack melted, are also suggestive. See Modern Water. Above left, gullies attributed to glacier meltwater.